When I first booted my computer, my RAM was detected at 1333MHz. So I loaded the XMP Profile in the BIOS. It switched my RAM frequency to 1600MHz and entered automatically the right timings. But when I rebooted the computer, it wouldn't boot at all. The MemOK red led was switched on, so something was wrong with the RAM settings. I had to push the Mem OK switch to run the Mobo RAM tests. The computer rebooted and I could finally access the BIOS. But the settings were changed and everything in the XMP profile was set to Auto instead of specific values like DDR3-1600MHz, or timings.

For some reason my RAM was running at 1600MHz with the frequency setting set to AUTO (wouldn't boot otherwise, Mem OK red led and so on) but it suddenly switched to 1333MHz, 2 days ago. I reloaded the XMP profile in the bIOS but couldn't the same problems as described before occured:- the computer won't boot- the Mem OK red led is switched on- have to manually push the Mem OK Switch so the mobo sets the RAM with values to its liking- the mobo switching most of the settings to AUTO

In CPUz I can see the RAM is running at 1600MHz but I'm 100% positive it'll switch again to 1333MHz one day or the other.

The simple explanation: Sandy Bridge IMC is 1333 MHz {667 MHz}, and if you placed those same sticks say in an i7 9XX Bloomfield then that value would read 533 MHz. You top image DRAM Frequency 'is' 1600; DDR Double Data Rate, CPU-Z displays 'I/O bus clock' instead of 'Data rate' which maybe not you but most are confused.

In most P67 {B3} posts I've been telling everyone to update their BIOS even prior to installing their OS. There have been a range of problems, and most of them are SATA related.

edit: The 'why' - if you look in your manual you'll see, as in most QVL, 'DIMM Socket Support' with 2-DIMM & 4-DIMM support listed with all sorts of 'kits' listed. Some are fine with 4-DIMM and some 'kits' are not {2-DIMM only}, and so it seems your 'kits' are 'not' which isn't a big deal - hence the manual. There's no advantage of XMP vs Manual, and if you OC then I don't recommend 2X__GB kits to use XMP. OC has a real bad habit of over-voltage to the DRAM Voltage.

You understood the line perfectly. Some BIOSes don't have exact values, so it might have 1.54v or 1.56v instead of 1.55v. Just use whichever is closest. You may even need to set it closer to 1.6v for it to work.

You were having 'Boot Failure' so the settings you listed aren't working. Running Prime95, Memtest86+ with 2X4GB only proves the RAM isn't the problem -- it's a BIOS problem with 4X4GB. The manual settings I provided should correct the problems incl XMP and/or voltage issues.

What I mean by {± closest value in BIOS); the values available in the BIOS DRAM Voltage 'menu' might list 1.546.... or 1.573... {1.546 is the closet value to 1.55v} which is a baby increase of 5/100's of a volt. Frankly, with 4X4GB 1.60v± wouldn't be surprising to me, and NO 1.55-1.70 won't harm your RAM. Damage is when RAM has DRAM Voltage +0.30~+0.40v over spec.

Worst is Auto settings when or if you OC your CPU, I've seen DRAM Voltage -> Auto add +0.30~+0.40v over spec and higher...

Thanks guys this is really helpfull. I'm wondering one thing: you're both recommending to set the tRAS value to 25 although my RAM timings specs indicate 9-9-9-24.What's the reason for this? Why would Gskill tRAS 24 sticks on the market if you have to set the value to 25 anyway?

CPU-Z shows a tRas value of 25, which is what is stored in Module. I would use that as opposed to writen Info. If info/Stored is in conflict, I normally select the Looser of the two value. Your not going to see any diff if set to 24 or 25 in terms of performance.

On my Ripjaw DDR3-1600 CL7 @1.6V. The timings use to be 7,7,7,7, But the newer ones now list 7,8,7,7. I added a set of 2 x 2 Newer ones to my set of 2 x 2 older ones. The new ones still showed the 7,7,7,7 timings and bot sets ran fine with that setting. Subsequently went with 4 x 4 CL=7,8,7,7 which is what it is set to - Absolutly no diff in performance that I can tell.

For what it's worth, I have my P55-UD4P set to use XMP profile 1 which set everything to Specs - But do concur manually setting is prefered.

Thanks guys this is really helpfull. I'm wondering one thing: you're both recommending to set the tRAS value to 25 although my RAM timings specs indicate 9-9-9-24.What's the reason for this? Why would Gskill tRAS 24 sticks on the market if you have to set the value to 25 anyway?

I did a short video showing what I do in the BIOS, so you can confirm I'm doing exactly what you told me to. The only thing I'm doing we can't see on screen is me pushing the computer "power" button to start up the machine at 1'32 (you can actually hear me pushing the switch)

You can see the computer won't be able to boot when I change the RAM settings. I have to push the Mem OK switch to reset the RAM settings and when going back to the BIOS it shows that everything was reset to AUTO.

Yes, 1.2v on the VCCIO is still in the safe zone despite turning red in your BIOS. You may also have to try 1.65v on the DRAM Voltage. I don't like telling people to run their RAM at 1.65v with a Sandy Bridge system, but it might help in this case.

Reality check - assuming all you're changing is the RAM settings and nothing else there should be no problems. I noticed your G.SKILL post, and if you look for F3-12800CL9D-8GBXL kits there seems to be a lot of problems.

edit: Yes you can try DRAM Voltage -> 1.60~1.65v and if it works then a +0.15v is not considered 'bad' and VCCIO -> 1.1~1.2v isn't a problem and VCCIO of 1.2v is the highest safe value.